1. Field
This application relates to communication networks and, more particularly, to a method and apparatus for ascertaining the performance of a network switch.
2. Description of the Related Art
Telephone switches are used to connect telephone subscribers to the Public Switched Telephone Network (PSTN). FIG. 1 illustrates an example of how a telephone switch 10 may be deployed in a telephone network. As shown in FIG. 1, telephone subscribers 12 are provided with access to the Public Switched Telephone Network (PSTN) 14 through the switch 10. Subscriber loops 16 having one or more segments 16a, 16b, connect the subscribers to the telephone switch 10.
When a subscriber seeks to make a telephone call, signaling on the subscriber loop indicates to the telephone switch that the telephone switch should establish a connection between the subscriber's subscriber loop and another line leading to the PSTN. Traffic on the PSTN may be handled in any conventional manner, such as by converting the signals from the subscriber loops into optical signals and multiplexing them over an optical transport network. There may be different options associated with the type of telephone call requested by the subscriber, such as to include a call waiting feature on the call, to include special billing provisions such as where the dialed telephone number is a toll free number, etc. Each of these is treated as a separate type of call by the telephone switch. Setting up a call may require a switch to process an origination message as well as many subsequent progress messages. Indeed, a typical call may include 20, 30, or more progress messages in addition to the origination message. The type of call and the number of legs or stages on the network may affect the number and type of progress messages.
Occasionally it is necessary to increase the switching capacity at one or more points in the network to enable the network to handle larger numbers of telephone calls. In selecting a switch for deployment on the network, one quality that is of particular interest is the maximum number of calls a switch can handle in a given period of time. Conventionally, switches have been rated as being able to handle a certain number of kcalls/hour (thousands of calls per hour) although other capacity measurements may be made as well.
The capacity of a telephone switch is the number of calls that it can set up and tear down in a given time period. The average length of the call (holding time) is not generally important since the telephone switch does not need to perform any additional operations on the telephone call once it is set up. The amount of processing time the switch takes to set up and tear down a call is referred to herein as the “Average Working Time” or AWT. A typical AWT for a large switch is in the order of a few milliseconds to 10s of milliseconds. The AWT may depend on the type of call being processed, whether individual call billing or other features are enabled, and several other factors. Generally, the AWT will be calculated as the amount of time the switch must spend processing the origination message and the subsequent processing messages for the call. A “call” as that term is used herein will be used to connote the origination message and subsequent progress messages associated with the origination message.
Unfortunately, it is difficult to predict the AWT for some switches. The AWT varies with call rate because of internal queuing or buffering behavior. For many switches, as the call rate increases, the AWT decreases asymptotically toward a minimum non-zero value. There are other effects too, all of which make it difficult to declare the capacity of a telephone switch without a direct measurement. Ideally, a network operator and network equipment vendor would inject traffic at high rates directly into a switch until it demonstrates (by refusing new calls) that its capacity is reached.
Unfortunately, as the switch design is advanced to handle larger and larger call rates, the equipment required test the switch up to maximum capacity becomes incredibly expensive. For reasons of cost, it is not practical to test every switch configuration at its maximum capacity. Sometimes, test equipment may be installed that generates calls at 10–20% of the maximum capacity. The maximum capacity of such a switch must be estimated by extrapolation from the test results or from data measured on similar switches. Other informed assumptions are also used to refine the capacity estimate. Well known mathematical functions with several parameters are often used to extrapolate the measurements taken under low traffic conditions to obtain an estimation of the switch's AWT at capacity. The parameters are determined by a “best fit” method but have low statistical accuracy at scaling from the low traffic rates to the estimated high traffic rates achievable by the switch. Given the inherent inaccuracies of these methods at extending the results obtained under low traffic experimental conditions, it would be advantageous to have a new method of ascertaining the performance of a network switch using low volume test traffic without requiring extrapolation and few assumptions.